1. Field of the Invention
The present invention relates to an SrTiO.sub.3 -based capacitor which has a high dielectric constant with stable temperature characteristics. More particularly, the present invention relates to the use of BaO and CaO as an oxide liquid infiltrant for the formation of second-phase dielectric layers at the grain boundaries of an SrTiO.sub.3 -based grain boundary barrier layer capacitor (hereinafter referred to as "GBBLC").
2. Description of the Prior Art
GBBLCs exhibit by high dielectric constants which result from their specific microstructure of thin dielectric layers between semiconducting grains. The preparation of GBBLCs is well known, as reported by M. Fujimoto and W. D. Kingery ("Microstructure of SrTiO.sub.3 internal Boundary Layer Capacitors During and After Processing and Resultant Electrical Properties", J. Am. Ceram. Soc., 68[4] 169-173 (1985)), and G. Goodman ("Capacitors Based on Ceramic Grain Boundary Barrier Layer--a Review", Advanced in Ceramics, vol 1, Grain Boundary Phenomena in Electronic Ceramics, 215-231 (1981)). According to these reports, GBBLCs are prepared via two process steps: i) preparation of semiconducting sintered bodies and ii) infiltration of an oxide liquid into the sintered bodies to form dielectric layers at grain boundaries. For the preparation of semiconducting sintered bodies, donor dopants are usually added to SrTiO3 powder compacts and the donor-added SrTiO.sub.3 compacts are sintered in a reducing atmosphere. The infiltration of an oxide melt in an oxidizing atmosphere then produces dielectric layers at grain boundaries of the semiconducting SrTiO.sub.3 grains. The infiltrants for GBBLCs in current use include mostly low melting point oxides, such as PbO, B.sub.2 O.sub.3, Bi.sub.2 O.sub.3, CuO, etc [Takahashi Yasunori, Yamaoka Nobutatsu, Yamaoka Yoshifumi, Kakubari; Shuichi, "Intergranular insulation type polycrystalline ceramic semiconductive composition", U.S. Pat. No. 3,933,668, Nobutatsu Yamaoka, "SrTiO.sub.3 -Based Boundary-Layer Capacitors", Am. Ceram. Soc. Bull., 62[8], 1149-1152(1986)].
The dielectric property of the thus prepared SrTiO.sub.3 -based GBBLCs is known to be determined by the infiltrant chemistry and the infiltrated microstructure. According to R. Wernicke ("Two-layer Model explaining the Properties of SrTiO.sub.3 Boundary layer Capacitors", Advances in Ceramics, vol. 1, Grain Boundary Phenomena in Electronic Ceramics, 272-281, 1981), the dielectric constant of SrTiO.sub.3 -based GBBLCs increases with an increase in the SrTiO.sub.3 grain size and with a decrease in the thicknesses of an infiltrated oxide liquid phase and an oxidized layer formed during the liquid infiltration. In addition, the higher dielectric constant of the dielectric layer formed between SrTiO.sub.3 grains, the higher is the dielectric constant of the fabricated SrTiO.sub.3 -based GBBLC.